1. Field of the Invention
The invention relates to a method and device for text detection in a video signal.
2. Description of the Related Art
European Patent Application No. EP-A-0,687,105 discloses a method for detecting motion in a video signal. The method can be used for generating new lines between the lines in a field. First, a difference signal across an image is calculated. This difference signal, which gives the minimum motion region of the location examined, is expanded horizontally using edge information calculated from two successive fields. The edge detector is based on a thresholded difference signal between successive odd or even-numbered fields defining the minimum motion region, and on a controlled horizontal expansion of the minimum motion region according to horizontal edges. Herein, a horizontal edge has a high vertical frequency.
U.S. Pat. No. 5,051,826 discloses a vertical edge detection circuit for a television image motion adaptive progressive scanning conversion circuit. The vertical edge detection circuit includes a first (line memory based) circuit for generating an intra-field difference signal from an interlaced scanning input television signal, and a second (field memory based) circuit for generating an inter-field difference signal from the interlaced scanning input television signal. A third circuit selectively outputs a maximum of the intra-field and inter-field difference signals when the inter-frame difference signal is less than a predetermined value, and the intra-field difference signal when it is greater than the predetermined value. What is called a “vertical edge” in this patent, is the same as what is called a “horizontal edge” in EP-A-0,687,105.
U.S. Pat. No. 5,565,998 (Attorneys' docket PHB 33.833) discloses a method of identifying film frames in a video sequence.
Known motion-compensated interpolation techniques appear to suffer from the problem that they are not capable of coping with scrolling text. Especially difficult is video mode text scrolling over a film mode background, which happens when video mode text is added to a film mode signal. Herein, film mode means that a pair of interlaced fields relates to the same motion phase, which occurs when the 50 Hz video signal is derived from a 25 Hz film signal by using 1 film frame to generate 2 interlaced video fields. In video mode, each field represents a separate motion phase. These problems are not dealt with in the above-mentioned publications.